1. Field of the Invention
This invention relates to an antenna array, and more particularly, to a method and apparatus for providing an auxiliary array antenna pattern that may be used to suppress interference in the received signal.
2. The Prior Art
A typical radar system employs a highly directional antenna which has a narrow main beam and many sidelobes. As is known, a signal within the received frequency band of the radar may cause radar jamming by having sufficient amplitude to saturate the radar receiver when that signal is incident on the antenna sidelobes as well as the main beam. Thus, the presence of sidelobes makes it possible for a single jammer to be effective against the radar from any angle of azimuth. In order to counter this prossible jamming, prior art systems have been developed which effect the cancelling or blanking of the interfering signal. These systems generally employ omni-directional auxiliary antennas having a substantially uniform gain as inputs to a cancellation system. The antennas are combined in a manner that will cancel the energy received in the main antenna sidelobes at discrete angular directions, thus reducing the interference energy received in the sidelobes and minimizing the deleterious effects of hostile jammers.
While such prior art cancellation systems are generally adequate, their effectiveness is somewhat lessened in radars which employ Moving Target Indicator (MTI) techniques. Specifically, when clutter energy in the direction of the radar's principal antenna main beam is received in an auxiliary antenna, the amount of the sidelobe cancellation is reduced.
It has been discovered that such interference clutter can be minimized by putting a notch or null in the auxiliary antenna pattern in the direction of the principal antenna main beam. Ideally, each of these auxiliary antennas generates a radiation pattern that receives little energy in the angular direction of the main beam of the principal radar antenna, but receives a relatively strong energy level of uniform amplitude at all other angular directions.
In theory, the provision of such a notch in the auxiliary antenna pattern minimizes the otherwise reduced effectiveness of sidelobe cancellation systems in MTI radars when clutter energy in the direction of the main antenna beam enters an auxiliary antenna. In practice, however, such provision presents additional problems that heretofore made such a method unworkable in phase scanning systems.
For example, one known method for providing auxiliary notch antennas has been by adding additional elements and support structure. However, this method overcomplicates the system and makes it unusable where space is at a premium. Another known method, for use in antenna arrays, has been the use of some of the array elements as auxiliary antennas. Such a method has not been viable, however, because it has inevitably decreased the gain of the full antenna and increased the sidelobe levels. In the case of electronically steerable arrays, additional problems exist. For example, there is a need to steer the notch in the auxiliary antenna pattern in the same direction as the main beam of the full array. In addition, providing a notched auxiliary radiation pattern in such an array typically results in the formation of grating lobes which in turn create additional notches in the auxiliary radiation pattern thus violating the desired design of providing a notch or null solely in the direction of the principal antenna main beam and a substantially uniform gain in all other directions. Further, the combining circuitry required for providing a notch in such an array is very complicated and in many cases the desired amplitude of the "out-of-notch" levels cannot be met across spatial angles of interest.
Accordingly, it is an object of this invention to provide an apparatus and method for improving sidelobe cancellation using an array antenna by providing a notched auxiliary antenna radiation pattern that employs existing elements and support structure, and which does not decrease the gain of the full antenna nor increase sidelobe levels.
It is another object of this invention to provide an apparatus and method for use in a series fed electronically scannable radar array antenna having the above characteristics and which produces a scannable precision tracking notch of desired width and level without in any manner disturbing the main beam or sidelobes of the array.
It is a further object of this invention to provide an apparatus and method for use in a series fed electronically scannable radar array antenna having the above characteristics and which includes means for controlling the depth and slope of the notch as well as the power level in the generally flat, "out-of-notch region" of the auxiliary radiation pattern.